br＞Background/br＞ Treatment outcome of childhood cancers have improved, resulting in overall survival rate greater than 70%. As childhood cancer survivors have long expectancy of life, second malignant neoplasm (SMN) is one of the most concerning late effects of childhood cancer treatment. We conducted this study to evaluate the incidence and factors related to second malignant neoplasm after childhood cancer in South Korea. br＞Materials and Methods/br＞ Cancer incidence data between 1993 and 2012 was obtained from the Korea Central Cancer Registry. Each cancer was classified into subgroups according to the International Classification of Childhood Cancers, 3rd edition (ICCC-3). The person-years at risks of observation were cumulated until the date of last known vital status, death or end of study, which was December 31, 2012. The risk of SMNs was calculated using standardized incidence ratio (SIR), excess absolute risk (EAR), and cumulative risk. br＞Results/br＞ Among 28,345 cancer patients aged 0-19 years, 336 individual (1.2%) were diagnosed with SMN. Median age at initial cancer diagnosis was 11 years. Total follow-up was 197,312 person-years, with a median follow-up of 5.6 years (0-19.5). The median latency between first and second cancers was 5.3 year and median age at SMN was 16 years. Childhood cancer survivors were at nearly 20-fold risk of developing a new cancer relative to the general population and overall EAR was 16.3 per 10,000 person-years. The SIR was higher in female (23.4) than male (17.6). Overall SIR was highest at the 10 year (32.8) after initial cancer diagnosis. And the cumulative incidence of developing a second cancer was 0.6% at 5 years, 1.2% at 10 years and 2% at 15 years. The highest risk of SMN occurred among patients previously diagnosed with soft tissue sarcoma (SIR=51.8) and malignant bone tumors (SIR=45.3). br＞Conclusion/br＞ This is the largest Korean data about SMN which is based on population registry. Previous data showed that cumulative incidence of SMN increased when the follow up period lengthened, and this tendency was also observed in our study. Thus, long-term follow up of childhood cancer survivors and further study is needed for more profound data.